Low temperature preservation is a widely used method for preserving food, and is also known to be an effective method. The technology of low temperature preservation can be broadly classified into refrigeration and freezing, and it is also widely known that from a microbiological viewpoint, lower storage temperatures are the most beneficial.
However, a problem associated with preservation methods that utilize freezing is the prevalence of deterioration in the quality of the product, known as damage by freezing, which includes the generation of drips, protein denaturation, and damage to meat as a result of damage to, or changes in, the cellular structure, all of which can be caused by freezing.
However, it is known that if a food is cooled under specific cooling conditions, then it can be converted to an unfrozen state even at a temperature below the freezing point of the food, namely, a so-called supercooled state, and when foods are preserved in this type of supercooled state, damage by freezing such as protein denaturation and cellular structure damage can be avoided.
One method of preserving food in a supercooled state is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 5-161449, wherein fruit and vegetables are sealed in a plastic bag with a thickness of 20 to 100 μm, cooled to a temperature that is 1 to 2° C. higher than the freezing point of the fruit and vegetables over a period of 1 minute to 12 hours, subsequently cooled to a temperature just higher than the disturbing temperature of supercooling at an extremely slow cooling rate of −0.5° C./hour to −0.5° C./24 hours, and then either stored or transported in that state.
Furthermore, Japanese Unexamined Patent Application, First Publication No. Hei 8-252082 discloses a method for producing a supercooled state by rapidly cooling food from room temperature to a temperature near the freezing point, and subsequently cooling the food further, to a temperature below the freezing point, at a slow cooling rate of 0.01° C./hour to 0.5° C./hour.
However, these methods require a slow cooling step in order to achieve a supercooled state, meaning not only is considerable time required for the cooling step, resulting in a low processing throughput, but adequately suppressing bacteria reproduction during the cooling step can also be difficult.
Furthermore, a supercooled state is a state in which phase transitions of liquids or gases do not occur even if cooling occurs below the phase transition temperature, and is consequently an extremely unstable state. As a result, the supercooled state can be disturbed very easily by the incorporation of impurities, vibrations, or temperature fluctuations, causing the food to freeze, and accordingly supercooling has been viewed as impractical as a method of preserving food.
In addition, converting water that contains impurities, such as tap water, into a stable unfrozen state is difficult, and in the aforementioned Japanese Unexamined Patent Application, First Publication No. Hei 8-252082, the method of producing unfrozen water required a step for removing impurities from the untreated water by filtering the water through a microfilter and/or performing a distillation.
However, providing this type of step for removing impurities from the untreated water increases the complexity of the production process, which causes a deterioration in productivity and an increase in production costs.
The present invention takes the above circumstances into consideration, with an object of providing a method of preserving food which enables the food to be converted to a supercooled state within a short timeframe, and also enables the supercooled state to be maintained with good stability.
Furthermore, the present invention also provides a method of preserving food which enables food to be preserved in frozen state with little deterioration in quality.
Furthermore, the present invention also provides a method of producing unfrozen water, which enables the unfrozen water to be produced by a simple process, at low cost.